The known microscopic phase contrast method allows to see object structures in cells which are invisible in the normal bright field. Thus, on the illumination side, only beams in the determined aperture region (generally annular) are selected for illumination of the phase object. On the image side, a cemented component consisting of, for example, two plane-parallel plates, is inserted at the location of the real pupil, and a specific structure is applied onto the cement surface so as to manipulate amplitude and phase. In doing so, zeroth order beams are attenuated, and the beams of higher orders receive a 90° phase jump. The attenuation of the light intensity at the annular pupil region is generally effected by metal layers, which in turn result in a strong reflection of the incident beam. Typical reflection values of the combined amplitudes/phase structures—hereinafter referred to as phase structure—are 50% and more.
For objectives with smaller image scales (less than 20×), the real pupil is located at the rear part of the objective where the beams are parallel or quasi-parallel. Where such axially parallel beams are nearly perpendicularly incident on the above-mentioned planar surfaces, in particular the annular region comprising metal layers with a high reflection value, scattered light is generated by reflection. This may have a strong impact on the image contrast, which is a frequent problem in microscope objectives for contrast methods.
JP 9197285 A1 describes a solution in which the scattered light is reduced by a curvature of the cemented surface. For this purpose, two lenses, namely a plano-convex lens and a plano-concave lens, are used. The light reflected at the phase structures then no longer passes back directly into the object but is expanded by the curvature of the cemented surface and is spread out over the entire object. Since the production of curved surfaces and the application of phase structures onto these curved surfaces is complex, the corresponding production costs are, in fact, very high.
Moreover, solutions are known wherein a phase plate consisting of two plane-parallel plates is installed in an objective such that said phase plate is inclined at an angle to the optical axis of the objective. However, due to the inclination of the phase plate, such objectives have a greater constructional length.